In order to determine the effectiveness of online advertising campaigns, companies have traditionally relied on tracking logged-in users across one or more devices. In particular, user activity may be tracked, for example, in cases in which a user is logged into a service, e.g., a social media service or an email service, where a unique user ID is used by the service to identify the user, regardless of the device being used. While the aforementioned mechanism may be useful in tracking logged-in users, the above mechanism is unable to track activities of users that are not logged-in to a service.